1. Field of the Invention
The present invention relates to a fixing device provided in an electrostatic recording system, e.g., a copying machine, a facsimile, a printer, etc., to fix a toner image transferred to a sheet of recording paper, and also relates to a heat roller for use in the fixing device. More particularly, the present invention relates to an improvement in the heat roller.
2. Description of the Prior Art
In an electrostatic recording system, for example, a copying machine, a facsimile, a printer, etc., a toner image transferred to a sheet of recording paper from a photosensitive drum of a developing device must be heat-fixed to the recording paper by allowing the toner particles to catch on the fibers of the recording paper. A fixing device used for this purpose comprises a heat roller 100 having a heat source and a press roller 200 coated with a heat-resistant resin material, as shown in FIG. 53. The heat roller 100 and the press roller 200 are disposed in opposed contact with each other. A sheet of recording paper 300 having a toner image transferred thereto is passed through the area between the heat roller 100 and the press roller 200, thereby fixing the toner image.
The most common type of heat roller is the indirect heating type in which a heat source, for example, an infrared lamp, a halogen lamp, etc., is incorporated in a metallic sleeve having good thermal conductivity. However, the indirect heating type heat roller has the problem that it needs a costly thin tube-shaped lamp and a thick-walled metallic sleeve having a surface smoothed with high accuracy, resulting in an extremely high overall cost. In addition, because of the indirect heating this type of heat roller cannot rise in temperature rapidly. To solve these problems, direct heating type heat rollers have spread recently in place of the indirect heating type.
FIG. 54 shows a typical direct heating type heat roller that comprises a heating member 101 which is a cylindrical insulator coated at its surface with a heating layer of an electrically conductive material, and a pair of strip-shaped sliding pieces 102 which are in contact with the outer peripheral surfaces of both end portions of the heating member 101, thereby supplying electric power to the rotating heating member 101 through the sliding pieces 102. FIG. 55 shows another typical direct heating type heat roller that comprises a cylindrical heating member 101 which is formed from an electrically conductive ceramic material in its entirety, and a pair of bearings 400 which are fitted on the outer peripheries of both end portions of the heating member 101, so that the heating member 101 is rotatably supported by the bearings 400 and, at the same time, electric power is supplied to the two end portions of the heating member 101 through the bearings 400.
However, this type of heat roller has various problems to be solved.
First, the heat roller that employs the sliding pieces 102 to supply electric power, shown in FIG. 54, has the problem that the surface of the heating member 101 is worn away by the sliding pieces 102 and sparks are given off at the area of contact between the heating member 101 and the sliding pieces 102. The heat roller in which electric power is supplied through the bearings 400, shown in FIG. 55, suffers from the problem that an electric conduction failure may occur because electricity flows through balls interposed in the area between outer and inner rings and the contact between the balls and the outer and inner rings is not always perfect
In addition, since the surface of the heating member 101 is heated to an exceedingly high temperature, the bearings 400 that are in contact with the heating member 101 are exposed to the high temperature, as a matter of course. Under such conditions, the bearings 400 wear at a remarkably high rate, and thermal expansion of the bearings 400 occurs. Accordingly, the rate of incidence of conduction failure is extremely high in the present state of art.
There are other forms of power supply in the direct heating type heat roller, in which electric power is supplied through metal terminals that are secured to both end portions of a cylindrical heating member by silver soldering or through metal terminals that are closely fitted in respective hollow portions at both ends of a cylindrical heating portion. However, the power supply method that uses metal terminals secured by silver soldering has the problems that the soldered portions cannot endure mechanical vibration that accompanies the rotational motion of the heat roller and that the soldered portions may separate during the process of use because of the difference in the coefficient of thermal expansion between the silver soldering material and the ceramic heating member. The method in which power is supplied through metal terminals that are closely fitted into respective hollow portions at both axial ends of a cylindrical heating portion has likelihood that since the metal terminals have a high coefficient of thermal expansion, when thermally expanded, the metal terminals may destroy the ceramic heating member In addition, a feed brush for supplying electric power to each conducting terminal must be provided in sliding electric contact with the free end of the conducting terminal, that is, with the end of the conducting terminal on the side thereof which is opposite to the end thereof that is fitted into the hollow end portion of the heating member. It is therefore necessary to control the positional relationship between the heat roller and the feed brushes extremely strictly In addition, the operation of assembling the heat roller and the feed brushes is difficult, so that the assembling efficiency is low.
Thus, the conventional heat rollers involve problems in terms of the structure for rotatably supporting the end portions of the heat roller heated to a high temperature and of the method of supplying electric power to the heat roller. In view of these circumstances, there has been a demand for a technique that enables good electric contact to be surely made between the feed brushes and the conducting terminals, which serve as feed means, so that electric power can be supplied to the heating portion effectively and reliably, and that also permits minimization of the heat load applied to the bearing portions.
There has heretofore been another problem in terms of the control of the surface temperature of the heat roller. That is, it is necessary in order to ensure effective fixing of the developer to maintain the heat roller at a predetermined temperature. In addition, an excessive rise in temperature should be avoided from the viewpoint of preventing a fire or other similar problem. As a means of preventing an excessive rise in temperature of the heat roller, there has heretofore been only one method wherein temperature sensors of good response are disposed in close proximity to both axial ends of the heat roller, and signals output from the temperature sensors are processed in an electric circuit provided separately, thereby limiting the electric power supplied to the heat roller or cutting off the power supply. However, such an excessive temperature rise preventing means uses costly temperature sensors and needs additionally an electric circuit for monitoring signals output from the temperature sensors and hence it is costly, so that the use of the excessive temperature rise preventing means causes a rise in the overall cost of the apparatus and also hinders realization of a reduction in the overall size of the apparatus. Under these circumstances, there has been a demand for an excessive temperature rise preventing means that is simple in structure and inexpensive and that needs an extremely small space for installation or requires no installation space.
We have already discussed problems which are experienced when electric power is supplied through bearings that hold a heating member. Meantime, even in a case where bearings do not serve as feed means, there are some problems attributable to the use of bearings. More specifically, bearings are arranged such that a plurality of steel balls are retained in the area between inner and outer rings, and if a direct heating type heat roller is supported with such bearings, since the bearings rotate in direct contact with the heat roller surface that reaches a high temperature of 200.degree. C. to 300.degree. C., they deteriorate at a remarkably high rate, and the toughness of the bearing material lowers rapidly, resulting in an interference with the rotational function of the bearings during the process of use over a long time and hence a failure to obtain the desired rotational operation. Heat-resistant grease that is employed for smoothing the rotation of bearings serves to deter deterioration of the bearings, but it volatilizes at a remarkably high rate under high-temperature conditions. Accordingly, the use of such heat-resistant grease cannot solve the basic problem of the deterioration of bearings. Incidentally, there are bearings called "oil retaining bearings", although such bearings have not yet had the experience of being used as means for supporting a heat roller. Oil retaining bearings, which comprise an annular sintered metal member that is impregnated with a lubricating oil, can be used for a long time without lubrication under ordinary temperature conditions. However, if oil retaining bearings are used as means for supporting a heat roller, since the heat roller is exposed to high temperature, the oil volatilizes at a remarkably high rate, so that the oil retaining bearings are expected to lose their bearing function in an early stage of use. Thus, oil retaining bearings cannot practically be used as heat roller supporting means. Under these circumstances, there has been a demand for a bearing structure which has substantially no deterioration during the process of use and ensures a smooth rotation of a heat roller even after a long-term service and which is simple in structure and inexpensive.
In addition, it is preferable that the temperature of the heat roller surface should be uniform over the entirety thereof, but it has been difficult for the conventional heat rollers to realize a uniform temperature distribution over the heat roller surface If the surface temperatue is uneven in the axial direction, the toner fixing condition also becomes uneven, causing a fixing failure However, since the axial end faces of the heat roller are in contact with the ambient air and the axial end portions of the heat roller conduct heat to the bearing members, the temperature lowers at the end portions of the heat rollers. Referring to FIG. 56, which shows a temperature distribution in the axial direction of the heat roller surface, the temperature curve is flat and maintains a substantially constant value in the range L at the center of the heating member 101, whereas in the range L.sub.0 near each axial end portion the temperature curve lowers gradually. Thus, the temperature curve has descending regions in correspondence with the two axial end portions of the heat roller. The end portions of the heat roller corresponding to the descending regions have a marked lowering in temperature and therefore cannot be used as fixing regions, so that only the portion of the heat roller which has a flat temperature curve, exclusive of the end portions corresponding to the descending regions, can be used for the fixing process (the portion being hereinafter referred to as "usable region L"). However, in the conventional heat rollers, the ratio of the length of the descending regions to the overall length of the heating portion is relatively high, and it is therefore necessary in order to ensure a predetermined usable region to employ a heat roller having a heating portion considerably longer than the usable region, which is an obstacle to the realization of a reduction in the overall size and a lowering in the cost of the apparatus. In view of these circumstances, there has been a demand for a technique which enables minimization of the descending regions in the temperature curve and permits enlargement of the usable region.
In addition, a typical conventional fixing device comprises a heat roller 100 and a press roller 200, as shown in FIG. 57. The press roller 200 comprises a metallic core 201 and a thick-walled resin layer 202 formed on the outer surface of the core 201 by using a heat-resistant resin material. The resin layer 202 may be formed from a silicone resin material or a silicone foam in its entirety, or it may be formed by coating a silicone resin material on the outer surface of a silicone foam. However, such a press roller needs to form a resin layer over the surface of a metallic core with a uniform thickness in both the axial and circumferential directions and hence requires a high processing cost. In addition, since the press roller also needs a mechanism for rotatably supporting the core, the apparatus is complicated, and the assembling cost is high. Since the press roller and the mechanism for rotatably supporting it have certain sizes, the reduction in the size of the fixing device is limited In addition, a fixing device comprising a heat roller and a press roller needs additionally a paper guide 500 for directing paper to the area between the two rollers, and the presence of this paper guide 500 is an obstacle to achievement of a reduction in the overall size of the fixing device and other peripheral devices. Under these circumstances, there has been a demand for a fixing device structure which enables a lowering in the cost and a reduction in the size of the fixing device, and which also eliminates the need for the paper guide 500.
FIG. 58 shows another conventional heat roller 100 in which the outer surface of a roller body 103 is coated with a resin layer 104 for adhesion, e.g., silicone rubber, for the purpose of improving the adhesion to recording paper and thereby enhancing the heat fixing effectiveness This prior art also has problems to be solved.
It is extremely essential for heat rollers to ensure a uniform surface temperature and improve the heating response. More specifically, if the surface temperature is not uniform, fixing of toner to recording paper becomes uneven, and if the heating response is low, a relatively long time is needed to raise the temperature, so that it takes a long time to warm up the heat roller, and the heat roller requires a long time to return to a predetermined temperature after the recording paper has passed therethrough. Accordingly, it is impossible to speed up the fixing process For this reason, it is desirable to coat the resin layer for adhesion such that it contacts the roller body uniformly over the entire surface In actual practice, however, there are irregularities in the surface of the roller body 103 due to the limitation of the surface machining technique, and minute air gaps 105 are formed between the roller body 103 and the resin layer 104, as shown in FIG. 59. The minute air gaps 105 obstruct the transmission of heat from the roller body 103 to the surface of the resin layer 104 and hence hinder uniform heating of the heat roller surface and improvement of the heating response. This tendency is particularly remarkable in the case where the roller body is a heating member made of an electrically conductive ceramic material.
In view of these problems, it has been a conventional practice to set the target heating temperature for the whole heat roller at a level a little higher than the lowest temperature necessary for fusing the toner, thereby enabling the toner to be effectively fused even at low-temperature portions which are locally present on the heat roller surface. However, if the heat roller heating temperature is set at a relatively high level, the electric power is wasted, and the deterioration of the heat roller is accelerated. In addition, it is necessary to employ costly peripheral parts which can cope with high-temperature conditions. The most significant problem is that since the heat roller is heated to an excessively high temperature, the toner becomes likely to weld to the heat roller surface, so that a fixing failure is likely to occur If a toner removing device that is placed in sliding contact with the heat roller surface is provided in order to prevent such a problem, the lifetime of the heat roller is shortened owing to friction occurring between the heat roller and the toner removing device Under these circumstances, there has been a demand for a heat roller which is designed so that the heat roller surface can be heated uniformly and the heating response is improved, thereby enabling an excellent fixing condition to be realized without inviting a waste of electric power and a rise in the cost of the peripheral parts.